1. Field of the Invention
The invention relates to filter-driers which are used in the suction line, between the evaporator and the compressor in a refrigeration or air conditioning system, for filtering and dehydrating the refrigerant and oil in their vapor states.
2. Review of the Prior Art
In air conditioning, refrigerating, heat pump, and hot gas defrost systems of the prior art, filters and driers are essential components for removing harmful contaminants and protecting the motor compressor, the heart of any such system, which is called upon today to withstand increasingly severe operational conditions because market conditions are requiring smaller and more compact systems and higher speed compressors which subject the unit to higher temperatures and pressures and tend to shorten its life and increase the danger of premature breakdown.
Because the electrical portion of the motor is in direct contact with the refrigeration circuit, the chemical environment to which it is exposed is of prime importance. In addition, it is subject to damage from solids which must be prevented from reaching the compressor. These materials, in spite of utmost care in assembling and cleaning out a system, seem to be always present and are frequently not dislodged until the system is initially started up. They are major contributors to hermetic motor burn-outs and are also the cause of mechanical damage to close-tolerance parts by abrasive action.
At the present time, a system sanitizing approach is used to isolate the motor compressor from three general categories of contaminants which play important roles in compressor failure. These are: (1) harmful soluble chemicals; (2) damaging liquids and solids; and (3) oxygen present in air as a non-condensable.
Liquid line filter-driers are used to remove a broad spectrum of soluble contaminants, which include water, acids, oil breakdown products, tars, resins, gums, and dirt of relatively large particle size. The resins and gums are adsorbed, and the dirt is filtered out. Such receiver driers are described in U.S. Pat. No. 3,118,288 and U.S. Pat. No. 3,785,164. They generally include a desiccant, such as a molecular sieve or alumina, and a flow directing means for forcing the liquid to pass through the desiccant.
A liquid line filter-drier for refrigeration systems is disclosed in U.S. Pat. No. 3,815,752 which combines an adsorbent bed and, downstream thereof, a pad of fiberglass within a conical or cylindrical wire form so that there is ample space within the enclosing shell for liquids to swirl around the fiberglass and enter it over a large area. However, the incoming liquid initially contacts a perforated plate containing the granular desiccant.
Another spring-loaded filter-drier for the liquid side of refrigeration systems is taught in U.S. Pat. No. 3,841,490. It comprises an adsorbent bed within a perforated canister and a plurality of fiberglass pads within a cylindrical spring. The canister and pads are spaced peripherally from the enclosing shell, to provide an annular flow channel, but abut shoulders of the shell at each end thereof. Flow therefore passes through the inlet corners of the canister, then through the annular flow channel, and finally through the filter media.
A bi-directional filter-drier for heat pump systems is additionally described in U.S. Pat. No. 4,125,469. This device comprises a cylindrical canister, which is peripherally spaced from the walls of the shell, and an outlet valve at each end, which is surrounded by an annular perforated screen which is closed against outward flow by a flap washer. Incoming flow moves through the flap valve, passes radially through the filter, and then moves axially through the hollow core of the filter to and through the outlet valve.
However, vapor-phase filters having very little pressure drop are being increasingly used in recent years. Such vapor-phase filters are designed to remove harmful particles that are too small for filter-driers to take out in the liquid line, in addition to materials which are present in the system beyond the point where the liquid line filter-drier is installed.
Used in conjunction with a liquid line filter-drier, a vapor-phase filter on the suction side of a system effectively isolates the motor compressor from finely divided steel and other metallic particles which are believed to be the major cause of motor burn-outs when carried to the windings by high-velocity suction gas. These offending particles include metal chips, solder flux, copper oxide, iron rust, carbon, corrosion solids, and the like which contribute to motor burn-out or cause compressor damage through abrasion. However, a suction-side filter will filter out gross quantities of the foreign materials as small as 5 microns (0.0002 inch) in diameter, with negligible pressure drop while permitting high rates of gas flow in the suction line. Such vapor-phase filters are necessarily large in volume as compared to liquid line filter-driers.
Compressor damage is also caused by slugging of refrigerant and oils which typically occurs when a refrigeration or air conditioning system has been idle for an extended period. The suction effect of the compressor, when starting up after such idleness, creates such a low pressure that both liquid and vapor are pulled out of the evaporator and reach the compressor unless a means is provided for separating the liquid from the vapor and accumulating the liquid until it can be gradually reintroduced into the system as needed in the form of harmless droplets mixed with the vapor.
Excessive quantities of liquid refrigerant dilute the oil, wash out bearings, and in some cases cause complete loss of oil in the crankcase of the compressor because of the high solubility of the oil in the refrigerant. Because compressors are designed to compress vapors, not liquids, such accumulations or "slugs" also can result in broken valve reeds, pistons, rods, crankshafts, and the like parts of a compressor.
Thus, a storage component in the form of an uprightly disposed cylinder is commonly added to the suction side of the refrigeration or air conditioning system to act as a reservoir for temporarily holding the excess oil-refrigerant mixture and returning it at a rate that the compressor can safely handle. Such an accumulator usually can hold from about one-half to about two-thirds of the oil-refrigerant mixture that is within the system.
However, in some refrigeration and air conditioning systems, accumulators are too small or are not installed even though needed. However, if a suction line filter-drier is added to the system, it will then be subject to the damaging effects of liquid slugs moving at a high velocity. Fiberglass, for example, can be literally torn and disrupted by such high velocity slugs. The surface of a fiberglass pad that receives the impact of fast-moving particles of metal and scale can also be rapidly plugged thereby. A need, therefore, exists to protect the relatively delicate fiberglass from impact of high-velocity particles and liquid slugs.
It is, therefore, desirable to interpose a filtering medium between the evaporator and the compressor that can, at least, disperse such liquid slugs into smaller droplets which will be harmless to the compressor while simultaneously providing filtering benefits with respect to particles and drying benefits with respect to moisture. Furthermore, it has been found that any filter, liquid line or suction line, tends to become plugged rather rapidly and abruptly as particles build up on its surface. The pressure drop across the filter-drier therefore tends to build up abruptly and to cause the air conditioning or refrigerating unit to go out of service with little or no warning. A means for providing a gradual build up that would furnish preliminary warnings to maintenance personnel is consequently desirable.
Designs of prior art filter-driers have generally sought to minimize inherent low efficiency with respect to pressure drop through the devices, because pressure drop in the suction line of a refrigeration or air conditioning system adversely affects the total system capacity and the cost of operation. In view of the limited space that is available adjacent to modern engines and refrigeration and air conditioning systems, both stationary and mobile, and the need for both filtering and drying, it is highly desirable to be able to combine the functions and the space requirements of a filter for very fine particles and the protective function of vapor state drying within the same device.